Final Report Evaluation of I-Pave Low Volume Road Design … · 2015-08-03 · Final Report...

www.ara.com

SOLVING PROBLEMS OF GLOBAL IMPORTANCE

© 2015 Applied Research Associates, Inc. 1


www.ara.com


Final ReportEvaluation of I-Pave Low Volume Road Design SoftwareMay 2015Michael I Darter, William R. Vavrik, Dinesh Ayyala

www.ara.com



Objectives1. Obtain all documentation for I-Pave as well as 1993

AASHTO procedure2. Review I-Pave procedure – does it follow 1993

AASHTO procedure?3. Examine inputs and program built-in default values for

new and rehabilitation design and unit costs for both flexible and rigid pavements

4. Examine equations used for calculation of ESALs, design thickness and life cycle costs

5. Evaluate whether inputs and default values are appropriate and comment

www.ara.com



Methodology – Tasks1. Proper use of 1993 AASHTO design equations for

calculating ESALs for new flexible and rigid pavement design and rehabilitation.

2. List of user-defined inputs and default or built-in values for design variables, source of data for each variable and their correct use in design equations.

3. Subgrade characterization – resilient modulus (Mr) for flexible pavement design and modulus of subgrade reaction (k-value) for rigid pavement design.

4. Procedure used to calculate the design thickness of HMA and PCC layers for new and rehabilitation design.

1. Introduction

www.ara.com



Methodology – Tasks5. Layer coefficient adjustment and other design

considerations for different distress levels entered by user for all types of rehabilitation design.

6. Basis for proposition of rehabilitation alternatives and thickness calculation procedure.

7. Calculations involved in life-cycle cost analysis and an assessment of default unit costs and other variables provide an unbiased and reasonable life-cycle cost estimate.

1. Introduction

www.ara.com



Methodology – ApproachTerminology – Default variables are those which are built into the software, shown to or hidden from the user and are assigned fixed values and cannot be changed (hard-coded)1. Identify user-entered and default variables used in each

I-Pave calculation procedure2. Develop a detailed sensitivity analysis matrix using a

range of values for each user-entered variable3. Reduce the complete matrix for I-Pave runs – reduced

sensitivity matrix, representative of the entire range of selected values

4. Compare results from I-Pave output and 1993 AASHTO design equations to achieve objectives

1. Introduction

www.ara.com



Overview of I-Pave Software2. I-Pave Overview

New Pavement Design Flexible & Rigid

Rehabilitation Design Flexible & Rigid

New Flexible DesignOutput Summary

New Rigid DesignOutput Summary

Life Cycle Cost Analysis Output

New Flexible Design

Life Cycle Cost Analysis Output

New Rigid Design

www.ara.com



Methodology – Complete Sensitivity MatrixTraffic variables were used to develop a full factorial matrix• Design life – 15, 20, 25, 30 years• Initial ADT – 500, 1000, 1500, 2000 • Percentage trucks – 5, 10, 15, 20, 25%• Growth rate – 1, 2, 3, 4, 5% (compound)Total = 4 X 4 X 5 X 5 = 400 sets of input valuesOther variables were randomly assigned values from a selected range.• Reliability – 50, 60, 70 or 80%• Terminal serviceability, pt – 1.9, 2.0, 2.1, 2.2 or 2.3• Base and stabilized subgrade thicknesses – Allowable • Subgrade Type – Suitable, Select or Unsuitable

www.ara.com



Methodology – Reduced Sensitivity Matrix• Complete sensitivity analysis matrix – 400 sets of inputs• Reduced sensitivity analysis matrix – 50 sets of inputs• Complete sensitivity matrix used to calculate HMA and

PCC thickness using 1993 AASHTO design equations, and life cycle costs

• Calculations using1993 AASHTO equations were performed in Excel®

• Reduced sensitivity matrix used as inputs in I-Pave and output values were recorded for comparison with the 1993 AASHTO calculations

2. Methodology

www.ara.com



Methodology – Reduced Sensitivity Matrix

Index No.

Y (years) ADT0 T (%) G R (%) pt Base St.SG Subgrade

Type

2 15 500 5 0.01 70 2 0 10 Suitable

9 15 500 10 0.03 80 2.2 0 8 Select

12 15 500 15 0.01 50 2 12 12 Unsuitable

25 15 500 25 0.04 80 2.2 12 0 Unsuitable

41 15 1000 15 0.05 60 2.4 10 12 Unsuitable

61 15 1500 10 0.05 50 2.2 4 0 Suitable

73 15 1500 25 0.02 50 2.2 4 0 Unsuitable

80 15 2000 5 0.04 70 2.1 8 8 Unsuitable

100 15 2000 25 0.04 70 2 6 8 Select

2. Methodology

Index Number – Refers to specific set of input values

RANDOMLYSELECTED

www.ara.com



Traffic Inputs and Calculation of ESALs• Load equivalency factor (LEF) is the average number of

ESALs per truck. • LEF used in flexible pavement ESALs calculation was

found to be equal to 0.644775 (Iowa SUDAS Ch. 5 –Pavement Thickness Design, Table 5F-1.06)

• LEF used in rigid pavement ESALs was not equal to recommended value from same reference

• Iowa SUDAS Ch. 5, Table 5F-1.06 recommends LEF of 0.76639 for rigid pavements, whereas the value calculated from I-Pave output was 1.114

• Higher LEF Higher ESALs (~ 45% higher), therefore greater design PCC thickness

3. New Pavement Design

www.ara.com



Iowa SUDAS Load Equivalency Factors3. New Pavement Design

www.ara.com



Structural Variables – Default Values• Elastic modulus of crushed stone base, layer coefficients

and drainage coefficient are recommended values in 1993 AASHTO guide (Part II, Section 2.1.3) and SUDAS specifications (Table 5F-1.05).

• Sc – 646 psi is mean value of Iowa DOT QMC samples calculated by MEPDG study for Iowa (Wang et al., 2008)

• Load transfer coefficient, J is fixed at 3.2, whereas AASHTO design guide uses different values for slabs with and without dowels.

• Iowa SUDAS recommends J = 3.1 for PCC thickness < 8" and J = 2.7 for PCC thickness > 8"

3. New Pavement Design

www.ara.com



Structural Variables - SubgradeThree types of soil in I-Pave as per Iowa DOT specifications (Iowa DOT Standard Specifications, Section 2102)• Select soil CBR = 7• Class 10 soil (suitable) CBR = 5• Unsuitable soil CBR = 3

3. Subgrade

www.ara.com



Structural Variables – Subgrade TypesSelect Soil• Cohesive soils: A-6 or A-7-6 soils of glacial origin, Proctor

density > 110 pcf, Plasticity index > 10• Granular soils: A-1, A-2 or A-3, Proctor density > 110 pcf,

P.I < 3.0Class 10 Soil• Normal earth materials like loam, silt, peat, clay, sand and

gravel• Suitable – Density > 95 pcf, AASHTO M145-91 Group

Index < 30• Unsuitable – Not satisfying above requirements

3. Subgrade

www.ara.com



Examination of I-Pave Subgrade Strength3. Subgrade

Resilient modulus Mr for Iowa subgrade soils were derived from MEPDG study by Ceylan et al. (2008)Mr values from the study were determined from laboratory testing of specimens at optimum moisture content, which is a requirement for MEPDG Mr for select (CBR = 7) and suitable (CBR = 5) soils in I-Pave are obtained from Ceylan (2008)Mr for unsuitable soils (CBR = 3) is assumed as 4,500 psi

Soil Type CBR R-value MR (psi) k (pci)Select 7 13 7385 246

Suitable 5 11 6489 216Unsuitable 3 - 4500 150

www.ara.com



I-Pave Subgrade Strength3. Subgrade

Mr values for subgrade soils with no stabilized layer

Source: Ceylan et al. for Iowa MEPDG Implementation “Characteristics of Unbound Materials for MEPDG, MEPDG Work Plan Task 5, Iowa DOT”, 2008

www.ara.com



I-Pave Subgrade Strength3. Subgrade

When stabilized subgrade is present, a composite Mr value is used which

• depends on the thickness of stabilized layer• does not depend on type of natural subgrade

www.ara.com



Discussion – Calculation of k-value from Mr

Equation for converting Mr to k-value for rigid pavement design from 1993 AASHTO design guide is

6. Discussion

30(psi) M(pci)k R=

Equation for converting Mr to k-value for rigid pavement design as used (also shown in soil data output) in I-Pave is

19.4(psi) M(pci)k R=

Current I-Pave relationship leads to lower subgrade strength by 35%, thereby leading to higher PCC thickness. Example: For Mr = 5000 psi, k-value is reduced from 258 to 167 psi/in.

www.ara.com



Design Thickness Calculation Procedure• Design thickness was calculated using 1993 AASHTO

equations and compared to I-Pave output for all sets of input values in reduced sensitivity matrix

• Both flexible and rigid pavement design thickness calculation follow 1993 AASHTO procedure

• Flexible Design: Thickness from I-Pave is identical to that calculated from 1993 AASHTO design equation

• Rigid Design: Thickness from I-Pave is slightly LOWER than that calculated from 1993 AASHTO design equation

• Final design thickness in I-Pave is rounded to nearest higher 0.5 inches

• The minimum design thickness for HMA is 3 inches and the minimum design thickness for PCC is 6 inches

3. Design Procedure

www.ara.com



AASHTO and I-Pave Design Thickness Data

Index No

Flexible Pavement Rigid Pavement

Actual Thickness, in

Rounded Thickness, in

Actual Thickness, in

Rounded Thickness, in

AASHTO I-Pave AASHTO I-Pave AASHTO I-Pave AASHTO I-Pave

78 3.248 3.248 3.5 3.5 4.548 4.356 6 6

163 6.629 6.630 7 7 6.065 5.851 6.5 6

170 2.885 2.885 3 3 5.926 5.901 6 6

274 6.185 6.185 6.5 6.5 7.184 6.904 7.5 7

278 2.315 2.316 3 3 5.477 5.317 6 6

3. Design Procedure

Shaded cells show different final design thicknesses for PCC

www.ara.com



Rehabilitation Design Procedure in I-PaveRehabilitation design can be performed in I-Pave for three pavement types:1. Flexible pavement (AC)2. Rigid pavements (PCC)3. Composite pavements (AC over PCC)Inputs for new pavement design are also required for rehab design. Additional inputs required are • Pavement age in years• Length of section to be rehabilitated, miles• Severity of distresses, specific to existing pavement type

4. Rehabilitation Design

www.ara.com



Rehabilitation Design Procedure Overview4. Rehabilitation Design

www.ara.com



Rehabilitation Design – I-Pave Output4. Rehabilitation Design

www.ara.com



I-Pave Rehabilitation AlternativesFlexible Pavements1. AC overlay of existing flexible pavement2. Milling of existing AC surface with AC overlay3. Milling of existing AC surface with PCC overlay (only

available for low severity of any distresses)4. Cold in-place recycling of existing AC surface with AC

overlay5. Full-depth reclamation of existing AC surface with AC

overlay6. Remove and replace existing pavement through

reconstructionComposite Pavements – All options as above, along with complete removal and replacement of existing AC surface


www.ara.com



I-Pave Rehabilitation Alternatives4. Rehabilitation Design

Rigid Pavements1. AC overlay of existing rigid pavement2. AC overlay of existing rigid pavement with longitudinal

joint patching3. Crack and seat existing PCC slab with AC overlay4. AC interlayer as bond breaker with PCC overlay (only

available for low severity of any distresses)5. Rock interlayer with AC overlay6. Rubblization of existing PCC with AC overlay7. Remove and replace existing pavement through

reconstruction

www.ara.com



Rehabilitation Design Procedure Findings• I-Pave shows three possible rehabilitation alternatives for

every design run• ESALs for all three pavement types are calculated using an

LEF of 0.644775 (same as new flexible design)• No effective SN for existing pavements in output summary• No life cycle cost analysis is provided for rehabilitation

design, hence it is not possible to compare alternatives• No output is obtained when all five distresses are selected

simultaneously for flexible pavements (with any level for each distress)

• PCC overlay only selected if low severity distress exists!


www.ara.com



Rehabilitation Design ProcedureThe rehabilitation design procedure used in I-Pave could not be verified using the data collected. Problems faced during the analysis were:• Existing distress types and severity have some impact on

rehabilitation alternatives and design thickness• No reference document cited in I-Pave• Equation used to calculate required thickness of the

overlay and layer coefficients are not given, and could not be determined from available information

• Major obstacle for analysis was precedence order of distresses: Which distress predominantly controls what alternatives are shown in the output?


www.ara.com



Flexible Pavement LCCAAsphalt binder unit price displayed in output and that used in calculation are different, resulting in lower unit price of mix

5. Life Cycle Cost Analysis

www.ara.com



Flexible Pavement LCCA5. Life Cycle Cost Analysis

Maintenance CostOutput shows maintenance cost as $2,700/yearCalculations showed that maintenance for flexible pavement is fixed at $55,000 for entire design period, irrespective of design period lengthSalvage ValueCalculating initial cost and salvage value per lane mileAverage initial cost of HMA = $60/ton, or $23,760/inSalvage value used in I-Pave = $1.65/yd2, or $11,616/in Observation: Salvage value as benefit is about 50% of the initial cost, resulting in low EUAC for flexible pavements

www.ara.com



Rigid Pavement LCCAPCC Initial Cost• Default unit prices are shown only for integer values of

PCC slab thickness• Final design thickness is rounded to nearest highest

integer for calculating PCC initial cost• A pavement whose design thickness is 6.5" therefore has

an initial cost corresponding to that for a 7" pavement.• Does unit price include dowel bars?Aggregate Base and Subgrade Stabilization Cost• Material unit costs and use in calculation are same as

those for flexible pavements


www.ara.com



Rigid Pavement LCCA• No salvage value for PCC• PCC demolition cost of $2/yd2 is also used in EUAC,

i.e. reconstruction is assumed after 20 years• Maintenance cost is calculated as $1,900 X Design Life (in

years). For 20 years, maintenance cost is $38,000• Maintenance cost is used in calculation as a benefit

rather than as a cost. This lowers the EUAC for rigid pavements.

EUAC Calculated by I PaveTotal EUAC = EUAC Initial - EUAC Maint. + EUAC Demolition

= $19,145.90 - $2,727.82 + $808.58= $17,227.10


www.ara.com



Maintenance Cost ComparisonDesign

Life (years)

Flexible Pavements Rigid PavementsTotal Cost

PW, $* EUAC, $ Total Cost PW, $* EUAC, $

15 55,000 4,607.16 28,500 2,387.35

20 55,000 3,696.86 38,000 2,554.20

25 55,000 3,158.53 47,500 2,727.82

30 55,000 2,806.06 57,000 2,908.10


* Values calculated using I-Pave output data, other values are shown in I-Pave LCCA output

Calculation of total maintenance costs for flexible and rigid pavements are inconsistent

www.ara.com



I-Pave Life Cycle Cost Analysis?

• LCCA is performed only over the design life and does not consider any future costs. This violates FHWA policy in that LCCA should be over at least 35 years • Life Cycle Cost Analysis in Pavement Design, FHWA 1998• Guide for Pavement Type Selection, NCHRP Report 703

• I-Pave does not conduct LCCA


www.ara.com



Discussion – Subgrade Resilient Modulus

• Mr values used in I-Pave were derived from MEPDG study (Ceylan et al.)

• In the MEPDG study, resilient modulus testing was conducted on specimens at optimum moisture content, which is the required input for MEPDG

• For 1993 AASHTO flexible pavement design, Mr must be an "effective" roadbed soil modulus (wet of optimum) which accounts for seasonal variation in moisture conditions (1993 AASHTO, Part II, Section 2.3.1)

• Von Quintus and Killingsworth (1993): “Mr should be representative of the more critical moisture condition measured during the year, i.e. higher moisture content”

6. Discussion

www.ara.com



Discussion – AASHO Road Test Mr

• Resilient modulus of 3,000 psi was used to develop the AASHTO flexible pavement design equation, as explained by the last two terms of the design equation

2.32 log10 Mr – 8.07 = 2.32* log10 (3,000) – 8.06 = 0• This value is consistent with laboratory tests conducted

on subgrade soil from the AASHO road test (1993 AASHTO, Part III, Section 5.3.4)

• “The use of a value greater than 3,000 psi is an indication that the soil is stiffer than the silty-clay A-6 at the Road Test site, and consequently will provide increased support and extend pavement life.” (1993 AASHTO, Part III, Section 5.4.5)

www.ara.com



SummaryFactors that affect bias in I-Pave design procedure 1. Subgrade resilient modulus, Mr for flexible design is unrealistically

high for I-Pave/1993 AASHTO design2. Minimum PCC thickness of 6" for low volume roads whereas

minimum HMA thickness is 3" 3. No salvage value for PCC4. Load equivalency factor (LEF) for rigid pavements is higher than

Iowa SUDAS recommendation5. Rounding off PCC thickness to next highest integer value for initial

cost calculation – uses unit price of 7” slab for a 6.1” design6. Methods for calculating total maintenance cost for flexible and rigid

pavements are inconsistent7. Rigid pavement EUAC equation is incorrect – maintenance should

be treated as a cost instead of benefit8. I-Pave LCCA does not meet the requirements of FHWA or

NCHRP for pavement type selection

Final Report Evaluation of I-Pave Low Volume Road Design … · 2015-08-03 · Final Report...

Documents

Transcript of Final Report Evaluation of I-Pave Low Volume Road Design … · 2015-08-03 · Final Report...